In recent years, there have been many developments that apply solar energy as an energy source, for example, the irradiance of the solar energy is converted into electrical energy or thermal energy for application. Taking a solar water heater as an example, an operation principle of the solar water heater is to absorb the irradiation energy of the sunlight for heating water. The solar water heater is configured with a solar selective coating for transferring the irradiation energy into heat.
FIG. 1A is a schematic diagram of a conventional solar thermal collector. FIG. 1B is an enlarged partial view of the solar thermal collector of FIG. 1 along a section line I-I′. There are different types of solar thermal collectors. FIG. 1A and FIG. 1B represent a common type of solar thermal collector that absorbs the solar irradiance through an absorber plate 12 to transfer the solar energy into heat. Then the heat is transmitted to the heat conducting tubes 14, which are distributed on the absorber plate 12, so as to heat up a heat transfer fluid (for example, water or other suitable fluid, which is not shown) in the heat conducting tubes 14. In other words, after the solar irradiance is absorbed by the absorber plate 12 on the solar thermal collector 10, the absorbed irradiance is transferred into thermal energy, and the thermal energy is transmitted to the heat conducting tubes 14 by thermal conduction through the welding junction between the absorber plate 12 and the heat conducting tubes 14. Then the thermal energy transmitted to the heat conducting tubes 14 is further transmitted to the heat transfer fluid in the heat conducting tubes 14 by means of thermal conduction and convection, so as to heat up the heat transfer fluid. Moreover, the absorber plate 12 of the solar thermal collector 10 is generally coated with a solar selective coating (not shown) capable of improving solar irradiance absorbing efficiency, so as to improve the solar thermal transfer efficiency of the absorber plate 12.
However, according to the above description, it is known that the solar thermal energy has to go through a long distance, by means of thermal conduction, to cross the absorber plate 12, and through the welding junction and the wall of the heat conducting tubes 14 before it is transmitted to the heat transfer fluid in the heat conducting tubes 14. In other words, the transmission path of the solar thermal energy is relatively long. Specifically, as shown in FIG. 1A and FIG. 1B, in a common type of solar thermal collector 10, which is configured with the absorber plate 12 and the heat conducting tubes 14, where the heat conducting tubes 14 are generally welded onto the absorber plate 12, the solar thermal energy is transmitted to the heat transfer fluid in the heat conducting tube 14 through the absorber plate 12, the welding junction between the absorber plate 12 and the heat conducting tube 14, and the wall of heat conducting tubes 14. In this way, the transmission path between the absorber plate 12 and the heat conducting tube 14 is relatively long (i.e. the thermal energy has to pass through the absorber plate 12, the welding junction, and tube walls of the heat conducting tubes 14 before it is transmitted to the heat transfer fluid), and since the absorber plate 12 and the heat conducting tubes 14 are in contact with each other only through welding points, the contact area in between is relatively small (an area A shown in FIG. 1B). Overall, the solar thermal energy cannot be effectively transmitted to the heat transfer fluid. Therefore, the heat conductivity and thermal efficiency of the solar thermal collector 10 is limited.
In the conventional solar thermal collector 10, the solar selective coating on the absorber plate 12 is generally applied before welding with the heat conducting tubes 14. Therefore, the welding energy causes damage on the surface of absorber plant 12 and reduces the irradiance absorbing efficiency. Moreover, the absorber plate 12 used by the conventional solar thermal collector is generally made of metal with high heat conductivity, such as Al or Cu, to avoid the thermal efficiency of the solar thermal collector 10 to decrease further.